Nuclear fusion describes the class of reactions that combine light atoms (Hydrogen, Deuterium, etc.) to form heavier atoms. For the lightest atoms, the binding energy per nucleon is a sharply increasing function of atomic number owing to the lack of shielding and Coulomb effects. Thus, combining (for example) two Hydrogen atoms to create Helium yields an enormous amount of energy. However, the energy barrier (Coulomb repulsion of the protons) to be overcome as the pre-fusion nucleons are brought closer together is substantial. The barrier is smaller for the heavier isotopes of Hydrogen, Deuterium (a proton and a neutron), and Tritium (a proton and 2 neutrons). Several large government research programs (e.g., U.S. Department of Energy, Oak Ridge National Laboratory; International Thermonuclear Experimental Reactor [ITER]) have been dedicated to achieving the control of fusion reactions on a scale large enough for the eventual purpose of providing relatively clean and abundant energy.
There are two major approaches (with each possessing several offspring which differ in the details) to achieving controlled sustained nuclear fusion, their difference hinging on the method of confinement of the high-temperature, high-pressure and highly-ionized reaction volume (the so-called ‘plasma’ state). Inertial confinement fusion relies on high-power lasers impinging on a target capsule, some portion of which ablates and yields an imploding shock wave inside the fuel portion of the capsule. Magnetic confinement fusion relies on strong dynamic magnetic fields to confine and compress the reaction volume. Both approaches have been explored (with large-scale government sponsorship) for the better part of 50 years, at least in the United States. Large fixed facilities exist and are being planned in the United States, England, France and Japan, among other countries. To date, the most spectacular result from any of these programs is the achievement of controlled fusion reactions. No net energy has been proven to be harvested from any of these approaches, though sustained nuclear reactions (specifically D-T fusion) have been achieved for as long as 5 seconds.
Collapsing Cavities
Collapsing spherical cavities can achieve high cavity wall velocities, high internal pressures and high temperatures. Many of the designs for the earliest fission bombs employed spherical implosion of the shock waves generated by spherical shaped charges of conventional explosives. Fusion bombs, in turn, utilized in many instances spherical implosion of the shock wave from a fission explosion. In these cases the generic mechanism is inertial confinement.
The preceding examples, while illustrating the usefulness of the concept of spherical implosion for energy focusing, also illustrate the problem of control and yield of such reactions. First, the energy generated by the implosion must be well in excess of the required energies for initiating the fusion reactions. Estimates for the required input temperature (of necessity uncertain to within an order of magnitude depending on fuel mass, density, etc) for sustained nuclear reactions are on the order of 107 K. Assuming such energies are achievable, if the size scale of individual implosions and targets is too large, control is lost. The yield is simply destruction. If, on the other extreme, the scale of individual implosions and targets is very small, control may be achieved, but sustainment and yield are sacrificed. The size scale must be somewhere in between bomb scale (too large) and the atomic scale (too small). As well, the eventual harvesting of useful energy from fusion will require a simple and inexpensive means for sustaining a multiplicity of reaction sites.
If one considers the case of a spherical cavity, filled with a mixture of gas and vapor, and collapsing symmetrically in a liquid, the relevant size scales are one to hundreds of microns for the expanded cavity, and hundreds to tens of nanometers for the collapsed cavity. If spherical symmetry is maintained, then one may estimate the energy available during a free collapse of a cavity. Scaling law analysis for an adiabatic ideal gas (plus some bubble dynamics) yields, for example, that a bubble collapsing from an initial radius of roughly 50 microns, and containing only 0.1% gas volume relative to the equilibrium state at the initial radius, would collapse to roughly 500 nm radius, with an internal temperature on the order of 104 K. If, however, the initial radius of the bubble is increased only by a factor of 2, the resulting temperature is increased an order of magnitude to 105 K.
There are claims in the scientific literature that such cavity collapse has been used to achieve fusion using acoustics to drive the cavity implosion. See for example Taleyarkhan R P, Cho J S, West C D, Lahey R T, Nigmatulin R I, Block R C: Additional evidence of nuclear emissions during acoustic cavitation. Physical Review E 2004; 69. While very different from the concept disclosed herein, it is important to note that such acoustic inertial confinement fusion has not won widespread credibility. Furthermore, even granting that nuclear fusion has indeed been achieved as claimed, both power scale-up and size scale-up are intrinsically difficult for acoustic systems, limited as they are by the requirements that the wavelength be much larger than the cavity radius, and by inherent nonlinear acoustic propagation saturation as power input to the transducers increases.
Droplet Impact and Droplets with Cavities
The formation and use of streams of liquid propelled by very high pressure to very high speeds is well known in the industry. Such streams are used, for example, for cleaning surfaces of dirt, contaminants and rust and for removing coatings such as paint. Some researchers have noted that non-steady streams are more effective than steady streams for these applications. In non-steady applications, the stream is broken up into slugs or drops. See, for example, U.S. Pat. No. 3,983,740 incorporated herein by reference.
The prior art in this area of technology has suffered from several drawbacks including, but not limited to, the need to use a complex driver, such as a high frequency ultrasonic generator, or the like, for causing formation of the slugs or droplets. Further, without a driver, other prior art devices are unsuitable for this application due to the fact that the distance required for a stream of liquid to break up into droplets is so long that the stream atomizes to a mist instead of forming droplets.
A considerable amount of pressure may be induced by the impact of high-speed liquid droplets (a method for producing such is described below) on hard target surfaces. The original purpose of creating such droplet impact was to deactivate viruses in animal-cell bioreactor harvest liquor for producing parenteral pharmaceuticals.
In order to further increase the internal pressure within the impacting drops, gas-filled cavities have been placed inside the droplets so that the very high implosion pressure of a collapsing cavity would add to the peak pressure of the impact of droplets without cavities. The challenge then became how to create cavities inside the very-high-speed droplets driven by ultra-high pressure in the nozzle plenum chamber. It has been suggested that one means to create cavities of vapor in the droplets is by violently mixing into the water an emulsion of very-small (microns) droplets of liquid propane or butane that is very volatile at 1 bar. The vigorous mixing will saturate the water with the propane at pn=pmixing. The selected liquid state must be above the critical point at the water's plenum pressure.
However, the emulsion technique described above suffers from a lack of control over the timing and size of the resultant cavities, which begin to form from the emulsified phase micro droplets within the host liquid as the pressure decreases when the jet exits the high-pressure nozzle. Further, such cavities contain the entire volume of vaporized volatile compound, and their ensuing collapse may be damped by the cushioning effect of so much vapor in the interior.